i don't run right now as i'm PG and fat! (and not fit enough to keep running throughout pregnancy) (mainly cos i've been PG and fat for about 3 years on and off now!!)
but I used to do quite a lot of half Ms.
I'd say that, if you were doing 6 miles 5 times a week for quite a while, you've got a good fitness base there.
And this week, keep your legs going, but don't actually 'train'.
So do run if you can, but at an easy pace, and not too far. So if you usually run 6 miles, do a bit less than that, and at an easy pace?
What you need to balance is the benefits of keeping legs 'ticking over', and over-doing it. I'd say probably don't run after thursday if the half M is on Sunday?
only my opinion, and am sure there are others that know more than me....